1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method. In particular, the present invention relates to a quantization processing that determines quantized values of a plurality of colors at the respective grid points in a table so that the quantized values are associated with each other, in a configuration where pieces of multi-valued data for a plurality of colors are quantized by using the table to which these pieces of multi-valued data are input.
2. Description of the Related Art
An error diffusion method has been known as a quantization for converting multi-valued data to binary data. This error diffusion method has advantage of providing high quality image. In recent years, in order to provide a more various gradation reproduction, an approach has been made to set quantized data to be three-valued or more data instead of the binary. In this case, a plurality of threshold value is set so that three-valued or more quantized data is obtained.
According to Japanese Patent Laid-Open No. 2003-224730, a method is disclosed in which, in a case of obtaining three-valued or more quantized data by quantizing respective multi-valued data for a plurality of colors such as cyan, magenta, and yellow, a lookup table is used to associate the quantization of respective the multi-valued data for the plurality of colors by error diffusion with one another. In particular, this method sets quantized values of the respective plurality of colors at the respective grid points of a table to use the table and thereby obtain quantized values corresponding to the multi-valued data of the respective colors. In the table, these quantized values have been corrected based on density regions of the respective perceived densities defined in the table. According to this method, a favorable visual characteristic can be obtained in an image printed by superposing a plurality of colors on one another.
In a printing apparatus such as a color inkjet printer, it is known that four basic types of cyan ink (C), magenta ink (M), yellow ink (Y), and black ink (K) are used to perform printing. Another printing is also known that uses, in addition to the above four colors, ink having a relatively-low color material concentration (light ink) such as dye and pigment is used to print an image of a further high quality, for example, an image in which a reduced granularity in a highlight part and a high density in a shadow part are both achieved. An apparatus also has been known in which sizes of ejected ink droplets are differentiated for the same color to print dots of a plurality of sizes so that a high-quality image can be printed similarly. Specifically, the above two types of apparatuses form images so that the highlight part (a low density region of a print density) is printed with dots having a relatively-low optical density (light ink dot or small dot) and a shadow part (a high density region of a print density) is printed with dots having a relatively-high optical density (dark ink dot or large dot).
A high image quality equal to that of silver halide photography is required not only in a color image but also in a monochrome image and in particular a gradation is important in a monochrome image. In the case of a configuration for forming a monochrome image by mixing C, M, and Y inks, slight variations of the respective C, M, and Y inks, for example, variation in relative landing positions of the inks and variation in ink droplet sizes, causes a disadvantage that color change in an image deteriorates quality of the image. In the case of a configuration where an image is formed by black ink only, no variation may be caused in the above-described relative landing positions and in the ink droplet sizes. Thus, this configuration is free from a deteriorated image quality such as the color change. From the viewpoint as described above, a printing apparatus has been proposed in which whether an image to be printed is a color image or a monochrome image is determined and a different ink type is used depending on the determination result. Specifically, in a case of printing a color image, C, M, and Y inks are mainly used and in a case of printing a monochrome image, Kink is mainly used. Furthermore, in a case of printing the monochrome image, similarly to the above-described color printing, dots having a relatively-low optical density and dots having a relatively-high optical density are used as the print region demands to achieve both of reduced granularity in a highlight part and a high density in a shadow part, thereby providing the printing of an image having a photograph quality. As one configuration realizing this, an apparatus has been known that uses black ink having a high color material concentration (black ink: K ink) and black ink having a low color material concentration (gray ink: GY ink).
FIG. 1 illustrates how to use K ink and GY ink in the printing of a monochrome image. In a highlight part near the lowest density level shown by input data (region I shown in FIG. 1), only GY ink is used to perform printing. As the input level increases from this region, the ejection amount of GY ink (output level of density) increases (region II). Thereafter, when the input level exceeds a predetermined value, the use of K ink is started and a relatively-high ejection amount of GY ink is maintained (region III). This can consequently arrange K dots in a scattered manner while maintaining the high GY density. A region where the level is almost maximum (region IV) mainly uses K ink only.
However, when the gradation expression is performed as shown in FIG. 1 where dark and light dots respectively formed of dark and light inks (K ink and GY ink) are used, the granularity by the dark dots may be conspicuous in the region III (middle gradation region). This disadvantageous granularity in the middle gradation region is solved if the difference in the concentration between dark and light inks is reduced. However, this reduction of the concentration difference in turn makes it difficult to achieve both of low granularity in a highlight part and a sufficient high density in a shadow part.
To solve the above problem, a configuration may be considered as shown in FIG. 2 in which dark, medium, and light dots are used or four levels or more of dark and light dots are used. This configuration improves the above-described problem due to the use of the two types of dark and light dots (granularity and density). However, in the region III (where the input of large GY dots is started as medium dots) and the region V (where the input of large BK dots is started as dark dots) in FIG. 2, the granularity by the start of the input of medium dots and dark dots may be conspicuous.
On the other hand, the above described Japanese Patent Laid-Open No. 2003-229730 discloses a technique in which a separation quantization table (CM separation table) is used in order to suppress the superposition of dots of different colors for reducing the granularity. If this technique is used for dark, medium, and light inks, dark, medium, and light dots can be prevented from being superposed on one another as much as possible.
However, in the case of the technique disclosed in Japanese Patent Laid-Open No. 2003-224730, with increase of the types of inks used in an apparatus causing a proportional increase of the types of multi-valued data to be quantized, the table size increases exponentially based on the number of types of the multi-valued data. Specifically, if the technique disclosed in Japanese Patent Laid-Open No. 2003-224730 is applied to a system using three types or four or more types of dark and light inks, a disadvantage of an enormous table size is caused.
It should be noted that the above described problem may be caused not only in a case of using the dark and light inks but also in a case of differentiating sizes of dot formed with same color of ink (that is, in a case of using large and small dots).